This invention relates to an antilock control system for vehicle wheel brakes. When the brakes of a vehicle are applied, a braking force between the wheel and the road surface is generated that is dependent upon various parameters including the road surface conditions and the amount of slip between the wheel and the road surface. The braking force increases as slip increases, until a critical value of slip is surpassed. Beyond this critical slip value, the braking force decreases and the wheel rapidly approaches lockup. If the wheel is allowed to lock, unstable braking occurs, and vehicle stopping distance on uniform nondeformable surfaces increases. Thus, stable vehicle braking occurs when wheel slip does not exceed this critical slip value. An antilock control system achieves stable braking and minimizes stopping distance by detecting incipient wheel lock. One criteria that is used to sense incipient wheel lock is excessive wheel deceleration and/or excessive wheel slip. Once an incipient wheel lock has been detected, pressure is relieved at the wheel brake. Upon releasing the brake pressure, the wheel will begin to recover from the incipient wheel lock condition. When the wheel has substantially recovered, brake pressure is reapplied. One criteria that is typically used to indicate wheel recovery is a positive wheel acceleration. Reapplication of brake pressure results in the wheel again approaching lockup and the wheel cycle process is repeated. Brake force and vehicle braking efficiency are maximized during braking by cycling the brake pressure around an optimum pressure for the particular road surface. This optimum pressure corresponds to the brake force generated while at the critical wheel slip value. Since the brake force is a function of wheel brake pressure and road surface conditions, the optimum brake force and the corresponding optimum brake pressure will change as road surface conditions vary. To optimize vehicle braking during a stop on a changing or non-uniform road surface, the antilock control system must be able to respond to each road surface and seek a new optimal pressure quickly to insure maximum braking efficiency.
While most systems execute wheel cycle control based solely upon vehicle motion parameters such as wheel slip and acceleration, there is a known system which measure the optimal brake pressure corresponding to the brake force generated while at the critical wheel slip value during the aforementioned wheel cycling. This system then utilizes the optimal brake pressure value as a target, operating to reapply to a fraction (i.e. 80%) of the optimum pressure and then increasing pressure gradually until reaching the optimum again, at which point the system senses another incipient lock and repeats the wheel cycle process. Because pressure is gradually increased from a significant fraction of the previously determined optimum pressure, the system can cycle brake pressure at or near the optimum value for substantial lengths of time while the optimal pressure value remains relatively constant. Thus, this type of antilock control system will provide maximum vehicle braking efficiency on uniform surfaces. However, if the surface coefficient increases above that which the system was previously operating on, the corresponding optimal pressure will also increase. Gradually increasing pressure from a value which represents a significant fraction of the optimal pressure for the previous surface, but which does not represent a significant fraction of the optimal pressure for the new surface, will produce a prolonged period of below optimal braking force. Until the pressure has increased to a value which is a significant fraction of the new optimal pressure value, the vehicle will not be braking efficiently, and stopping distance will be compromised as a result. Thus, it is desirable to respond quickly to an increase in the surface coefficient, reaching the new optimal pressure before braking efficiency decreases.